A system protection scheme against operation risk faced by the ultra-high voltage direct current(HVDC)without corresponding direct power supply is designed

which can resist the over-frequency risk under the HVDC blocking fault. Double-layer structure of system protection covering wind

hydro and thermal power of sending-end power grid is put forward. The multi-object oriented real-time information communication multiplexing technology is studied to meet the communication requirements between master station and multiple execution stations. According to the characteristics of different types of power and its influence on the safe operation of power grid

the DC modulation and global optimization control strategy of wind

hydro and thermal power is presented. The simulation on a real power system shows the effectiveness and practicability of the proposed system protection strategy.